diff --git a/changelog.d/623-takeup-start-date.md b/changelog.d/623-takeup-start-date.md
new file mode 100644
index 000000000..b0247225e
--- /dev/null
+++ b/changelog.d/623-takeup-start-date.md
@@ -0,0 +1 @@
+- Date the Marriage Allowance take-up rate parameter from 2015-04-06 (when the allowance came into force) rather than 2000-01-01, document that 0.5 is the post-2019 steady-state value, and link the HMRC Marriage Allowance statistics page.
diff --git a/policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml b/policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml
index 5a21633ac..3afd65a48 100644
--- a/policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml
+++ b/policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/takeup_rate.yaml
@@ -1,11 +1,13 @@
-description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible couples (~50% take-up). The matching parameter in policyengine-uk-data (`parameters/take_up/marriage_allowance.yaml`) is the load-bearing one for microsimulation, applied stochastically when populating `would_claim_marriage_allowance` per person; this file should be kept in sync.
+description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible couples (~50% take-up) at steady state; take-up was materially lower in the first few years after the allowance was introduced in April 2015 (HMRC outturn costs ran consistently below original forecast for this reason). 0.5 is therefore the post-2019 steady-state rate. The matching parameter in policyengine-uk-data (`parameters/take_up/marriage_allowance.yaml`) is the load-bearing one for microsimulation, applied stochastically when populating `would_claim_marriage_allowance` per person; this file should be kept in sync.
 values:
-  2000-01-01: 0.5
+  2015-04-06: 0.5
 metadata:
   unit: /1
   label: Marriage Allowance take-up rate
   reference:
-    - title: Income Tax Act 2007 s. 55B
+    - title: Income Tax Act 2007 s. 55B (inserted by Finance Act 2014 Sch. 2)
       href: https://www.legislation.gov.uk/ukpga/2007/3/section/55B
     - title: House of Commons Library — Income tax allowances for married couples (SN00870)
-      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
\ No newline at end of file
+      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
+    - title: HMRC — Marriage Allowance statistics
+      href: https://www.gov.uk/government/statistics/marriage-allowance-statistics
\ No newline at end of file